We made an apparatus for measurement of mechanocaloric properties of rubber. It consists of a pair of rubber tapes put together and suspended in a temperature-stabilized space. The upper end of the sample is connected to a force gauge and the lower end to a motor-driven mechanical device that stretches the sample quantitatively and reproducibly. The temperature of the sample is measured with a thermocouple inserted between the rubber tapes relative to a reference point fixed in the sample space. The tensile force, extension and temperature are measured to a precision of 0.01 N, 0.1 mm and 0.05 mK and recorded on a PC at a sampling interval of 0.5 s. Experimental results on a natural rubber sample gave the entropy of extension and stress-strain relation up to 200% elongation. A negative term in the entropy of extension is given explicitly in distinction to the normal positive one. Further analysis based on the ideal rubber equations gave consistent values of the structural parameter characterizing the rubber. Energy dissipation taking place in the deformation, a correction term in the present analysis, has been determined quantitatively for a systematic study of the irreversible effects in rubber.